Giardiasis is a common waterbone intestinal disease and is caused by the flagellated protozoan Giardia lambia. The trophozoite, or motile stage, attaches to the microvillus border of the host intestinal epithelium by an attachment organelle called the ventral disc. The attachment of the trophozoite to the intestinal epithelium has been suggested to lead to the symptoms of giardiasis which include diarrhea and malabsorption. Recent evidence shows that the specific activity of enzymes found on the apical surface of the intestinal cells in experimental animals and man is reduced when trophozoites are present. The organism is transmitted via a cyst stage. After ingestion of the cyst the organism actively leaves the cyst to establish a new infection. The purpose of the studies proposed here are to elucidate the mechanism of trophozoite attachment, study lysosome-like vacuoles of the trophozoite which may affect the host epithelium and to determine the mechanism of excystation. The specific objectives of this proposal are: 1) to investigate the cytoskeleton of the attachment organelle by detection of actin filaments by TEM, 2) to investigate the cytochemical localization of lysosomal enzymes in the trophozoite at the fine structural level, 3) to determine the ability of the trophozoite to sequester foreign substances, including host glycoprotein, 4) to determine the mechanism of excystation by cytochemical localization of acid hydrolases, immunocytochemical localization of contractile protecins, and in vitro excystation with use of inhibiting agents of lysosomes and constractile protein.